Permeability tuned image attenuation circuits



w. F. SANDS 2, 49,148

PERMEABILITY TUNED IMAGE ATTENUATION CIRCUITS Sept. 14, 1948.

Filed Dec. 30, 1942 MAM/w E 84/1 05.

INVENTOR BY/gg/wwv ATTORN EY Patented Sept. 14, 1948 STATES re-r @FFIYCE PERMEAJBILITYTUNED IMAGE ATTENUA 1 'TION CIRQUITS William Francissands', West .Collingswood; N. l, assignor to Radio Corporationjofiimerica, a corporation of Delaware Application December 30, 1942, Serial'No; 470,543 it 4-Claims. (C1. 250-20) lllhenpresent invention-relates to image attenuationdcircuits in superheterodyne receivers,

anid imoreiparticularly to :such circuits utilizing permeabilitymuning.

It is welllknown that thesuperheterodyne type of sreceiver.iiszsusc'eptible to thereception of-an undesiredmr;interfering frequency alongwith the desired 'isignal ifrequency for :each setting of the receiver tuningrmeans. The undesired "frequency is commonlyrreferred toias the "image frequency.

and is removed from the desired signal by'twice the value of the intermediate frequency. This is because for eachzvalue-of .the local oscillator frequency, there are two frequencies which are spacedwne-above andthe other belowthe oscillater frequency-by the amount of the inter-t mediate frequency, each of which heterodynes with the localoscillator frequency .to. produce the. :same intermediate frequency. Various schemesihave beenzdevised forthe suppression or attenuation of .the image frequency.

Inaccordancevwith the present invention, there is provided-between the .antenna or .signal :collector and theifirst :tube .of the receiver; or between -.twosuccessive tubes, ea. couplingwsystem functioningias a iband pass :network so designed.

asto-effect: maximum transmission: at .the. desired frequency and to provide maximumiattenuation.

to the iin-terferingsimage frequency.

Briefly, the invention contemplates the use,- in

conjunction with a permeability tuned signal frequency circuit of a superheterodyne receiver, of a low impedance winding in coupled relation with the inductance winding of the signal circuit and I.

a capacitor connected between the highg-potential enus. of said windings. ance'loetweenith'e windings ismade of such magnitude' and sigziQth'at together with the capacitor a'parallel' trap vcircuit is formed which rejects the 'image'ffrequency. .By having theilow impedance winding .iwound coaxially and substantially coextensively over a greater portion or the.

entireiength of the .main tuning coil and with a varyingpitch, the rejection of'the image .frequency is made .to follow or track the tuning of .the receiver .across the signal frequency band. In order that the image rejection may be made substantially iin'finite a resistance cancellation element is employed (as shown by Bode, UfS'fPaterit No: 2;0i02 ,'216) .'to" balance out'the energy dissipatidninthe reie'ctor circuit;

"It'isoneio'f the 'main objects of thepresentin vention' thereforexito 'providexsimple signal 'fre quencymircuitsifor fuse in broadcast or short wave permeabilitytuned :receivers, having sub- The mutual .inductlzi stantially infinite image attenuation without; causing disturbance to'the normal receiver align merit, .and such that the. pointuof substantially infinite attenuation. remains tracked with'lth'e tuning of .the receiver across the ventirejtuningi" range.

Another object l, is to. provide .a per bi it tuned ,loop antenna circuit .having almost in finite. image attenuation.

These .and [other objects .andcadvantages .willv be apparent from .the following specification,

when taken with the accompanying .drawing;;'in'- which I Fig.1 .ldis'closes a ,permeabil'itytune'dsignal Ire" quency. circuit .in accordance with the invention providing image frequency attenuation,

Eig. .21shows the cirouit.of;Fig; ,1 exceptthat thepermeability tuned {circuit has -been irepl'aced f I by its velectricallequivalent mnetwork.

.FigLBlis a circuitembodyingrthe' principl of image rejection utilised in Fig. 1 but employing. two permeability tunedcircuits coupled b'ymeans of a link circuit;

Figs. 4 and 5 are image'attenuation circuits in accordance with the invention for use with a input circuit is tunablepverits frequency range by means of ferromagnetic core'means such as an iron core K movable. within the coil L2 in a known manner: a .A' low impedance winding L1 of relatively few turnsrlis wound coaxially over the :coilLz and in the same clirection'but with airelative'ly different distribution of turns .along the pathio'f Jsaid" core means. Preferably theco'il' L1 "is "wound with varying pitch, extendin lover the entire length of the coil Li oronlyovera subsstantialpart thereof. The two windingswhichin effect constitute Latransformen-with L1 as the 1 primary and L2 as the secondary, "have their low radio frequency potential tends connected together and which maybe connected directly to :ground or through a variable resistance 1. The:

high radio frequency potential ends of the windings have connected between them an adjustable capacitor C3. The received signal energy which may be derived from an antennaAj 'orjfrom'ia precedihgnadib "frequencyamplifier tube (not quency band of the receiver and to form with the condenser C1 a capacitive voltage divider from which the signal energy is fed to the primary L1.

An important feature of spreading the relatively few turns of coil L1 over the length of coil L2 or a greater portion thereof is that the normal alignment of the input circuit with the other tunable circuits of the receiver at the signal frequency is not materially disturbed, as would be the case if the turns of coil L1 were closely wound at one end, for example, the low potential end, of the secondary L2. By varying the pitch of L1, it is possible to keep the point of rejection "tracked across the entire range. By way of example: for a signal frequency range of 6-18 me. (i. e., 311) the image frequency would only vary from 6.91-18.91 mc., or a ratio of 2.74:1, and a slight amount of slowing up in the pitch is required. The foregoing has been based on the assumption of equal coil diameters for the signal and image frequency windings. In actual practice, the coil L1 would be wound over, and spaced from, the secondary L2 and some of the required slowing up would be secured also by this means. The series resistor r, in the circuit, may be adjusted for cancellation of dissipation in the mutual element, as described in the above mentioned Bode patent.

The above described coupling network of Fig. 1 may be resolved into its equivalent 11' network shown in Fig. 2 which is of the general form of band-pass networks with one point of infinite rejection, which in accordance with the present invention is tuned simultaneously with the tuning of the signal frequency circuit. The series arm of the 1r network is constituted by a parallel resonant circuit S which includes the capacitor C3 and an inductance having the value .the latter existing by virtue of the mutual inductance between the windings L1 and L2. The magnitude and sign of the series inductive arm of the equivalent network, and the value of capacitor C3, are such that the circuit S is tuned to the image frequency (,fs-l-2 I. F.) at any position of adjustment of the tuning core K. It is necessary that the inductance L1L M Lg-M L TZE L M as indicated by Fig. 2.

Byvirtue of the coaxial arrangement of the two windings, and the variation of the pitch of and 4 the primary along the length of the secondary, the inductance of the rejector or trap circuit S is varied in the proper sense with adjustment of the tuning core so that the rejection of the image signal is tracked across the entire tuning range. A modification of the circuit of Fig. 1 is shown in F. g. 3 which has the advantage that additional adjacent channel selectivity is obtained. The

essential difference is the use of two permeability tuned transformers T1 and T2 of generally similar construction as the transformer of Fig. 1, with their primaries L1 connected together to form a link coupling circuit. Each primary L1 is suitably spaced in a radial direction from its corresponding secondary L2 and is wound with the proper variation in pitch to secure tracking of the image frequency. The coil L2 of the first transformer T1 together with the antenna capacity C, the coupling capacitor C1 and the shunt capacitor C5 form a circuit which is tuned by' means of the core K across thedesired signal frequency range. Similarly, the coil L2 of the second transformer T2 together with the shunt capacitor C4 form. a circuit which is tuned by means of its core K across the same frequency range. The coupling circuit of Fig. 3 may be resolved into its equivalent 1r network which would be of the same form as shown in Fig. 2

except for different values of the series and shunt arms. The value of the series arm would be positive and equal to which together with the condenser C3 effectively in parallel therewith form an image rejector circonsideration is that the same arrangement be used for both transformers. The reason for this may be readily seen from the fact that when one transformer is wound with L1in the 'same direction as L2 and the other transformer with L1 and L2 wound in opposite directions, then the series arm of the equivalent vr networlgfor the two transformers, is of negative sign, i. e.,

For either of the preferred arrangements, stated previously, the series arm of the. equivalent 1r network for the two transformers is of positive sign. A third possible combination is that in which the primary of one transformer is wound in the same direction as its secondary, and the primary of the other transformer is wound opposite to its secondary. The proper sign for the inductance element of the image trap is secured by reversal of the leads of one of the L link windings.

An adaptation of thelmage suppression circuit of Fig. 1 to a loop antenna is shown in Fig. 4. Similar elements in both figures are designated by like reference characters. The high potential side of loop L1 is connected directly to the high side of the primary L1 which is chosen of such value to secure either an optimum, or a fairly uniform, transfer of voltage across the tuning range, as desired. .The method of operation of of Fig. 1'. The circuit of Fig. 5 differs from that of Fig. 4 in the respect that the resistance cancellation element r is connected between the low sides of the windings and ground.

In the case where it is desired to obtain a fairly large, but not substantially infinite, value of image attenuation (0.1), the resistance cancellation element 1 may be omitted, wherever shown. In Fig. 6 there is roughly shown by the solid line curve I the substantially infinite attenuation obtained where the resistance cancellation principle is employed, and by the dashed line curve F the large but entirely finite attenuation secured when the resistance cancellation element is omitted.

While I have shown and described certain preferred embodiments of the invention, it will be understood by those skilled in the art that modifications and changes may be made without departing from the spirit and scope of the invention.

What I claim is:

1. An input system for a superheterodyne radio receiver tunable over a range of frequencies and adapted when tuned to receive any desired signal frequency within said range to suppress a corresponding image frequency, said system comprising first and second coupled inductance windings having their low radio frequency potential terminals connected together and to ground, said windings being wound coaxially in the same direction to substantially the same length and having inductances L1 and L2, respectively, and a mutual inductance M, said first winding having a varying distribution of turns along said winding, capacitance means in shunt with the second of the windings forming therewith a resonant circuit, a ferromagnetic core movable in a path with respect to said windings for tuning the circuit to said desired signal frequency, and series capacitance means including a capacitor connected between the high potential terminals of said windings forming with the inductance of the series arm of the equivalent 1r network for the pair of windings a rejector circuit which, by reason of said distribution of turns, is tuned at each core adjustment to the corresponding image frequency, said inductance having a value of L1L,,-M M

throughout said range.

2. A circuit in accordance with the invention defined in claim 1 wherein the low potential terminals of the windings are connected to ground through resistance means which serves to reduce the energy dissipation in the rejector circuit.

3. A circuit arrangement for the suppression of the image frequency response in superheterodyne receivers, comprising a transformer having coaxial primary and secondary windings which have a common low potential terminal connected to ground, a capacitance connected between the high potential terminals of said windings, a resonant circuit including said secondary and a shunt capacitance, and a ferromagnetic core movable relatively to said windings to tune said circuit to any desired signal within a range of frequencies, said primary being wound in the same direction as the secondary and with. such varying pitch that at any position of said core the inductance of the series arm of the equivalent 1r network of said transformer forms: with the first-mentioned capacitance connected thereto an image frequency rejector circuit which tracks with the tuning of the resonant circuit.

4. A circuit in accordance with the invention defined in claim 3 wherein the low potential terminals of the transformer are connected to ground through a resistance which serves to reduce the energy dissipation in the image rejector circuit.

WILLIAM FRANCIS SANDS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,896,065 Budenbom Feb. 7, 1933 1,613,952 Johnson Jan. 11, 1927 1,703,079 Beers Feb. 19, 1929 2,051,012 Schaper Aug. 11, 1936 2,052,703 Farnham Sept. 1, 1936 2,137,475 Mountjoy Nov. 22, 1938 2,187,805 Landon Jan. 23, 1940 2,188,964 Sinninger Feb. 6, 1940 2,242,330 Schaper May 20, 1941 

